1. Field of the Invention
The present invention relates to a wireless communication apparatus and wireless communication method.
2. Description of the Related Art
The speed of a wireless LAN method standardized as IEEE802.11 has increased with the advent of, e.g., IEEE802.11b and IEEE802.11g. Presently, IEEE802.11n exceeding 100 Mbps is being standardized.
In IEEE802.11n, the use of the MIMO (Multi-Input Multi-Output) transmission method is being examined as a means for achieving large-capacity data transmission.
This MIMO transmission method is the technique by which both the transmitting side and receiving side comprise a plurality of antenna elements, and form a plurality of logical spatial streams (unique paths) by space-division multiplexing, thereby increasing the transmission capacity and communication rate.
Various methods can configure MIMO transmission. One known method is the SVD-MIMO method that uses SVD (Singular Value Decomposition) of a transfer function.
FIG. 3 is a view showing the concept of the SVD-MIMO transmission method.
Reference numeral 301 denotes transmission channels when the number of transmitting-side antennas is M and the number of receiving-side antennas is N. In this case, the channel characteristics are represented by an N×M numerical value matrix (to be referred to as a channel response matrix hereinafter). An element anm of this channel response matrix is a transfer function from the mth transmitting antenna to the nth receiving antenna.
Reference numeral 302 expresses an equivalent circuit obtained by performing SVD on the channel response matrix 301. EtH indicates the eigenvector of the transmitting-side antennas, Er indicates the eigenvector of the receiving-side antennas, and                √{square root over (λ1)} √{square root over (λ2)} . . . √{square root over (λM0)} (M0; the value of a smaller one of N and M)indicates virtual transmission channels (unique paths or spatial streams). By respectively presetting the eigenvectors EtH and Er on the transmitting side and receiving side, M0 signals can be transmitted with no interference from the transmitting side to the receiving side. Note that the amplitude gain of the unique path is        √{square root over (λi)}The transmission capacity changes in accordance with the size of the unique path.        
To perform wireless transmission using the SVD-MIMO transmission method, the eigenvectors EtH and Er must be respectively preset on the transmitting side and receiving side.
To obtain the eigenvectors EtH and Er on the transmitting side and receiving side, the transmitting side and receiving side must execute a calibration process before communication. Various methods and procedures of the calibration process have been proposed (US2005-094741 (corresponding to Japanese Patent Laid-Open No. 2005-142715), US2005-095996 (corresponding to Japanese Patent Laid-Open No. 2005-160030), and Japanese Patent Laid-Open No. 2006-33658).
The above patent references describe the procedures and methods of the calibration process itself, but none of them refers to the execution timing of the calibration process.